Fundamentals of Plant Anatomy and Physiology Related to Water Use - Introduction to Phytoremediation of Contaminated Groundwater - page 79

guard cells, which are specially shaped epidermal cells.
When water is limiting, guard cells shrink and remain
closed. When water is available, the guard cells swell and
open, permitting gas exchange to occur. The uneven length
of guard cells on adjoining sides of the stomata means that
when the water potential is high, the cells expand, and pull
apart, creating a pore that permits gas exchange to occur.
When water is not available, the guard cells shrink and the
stomata close, causing gas exchange to cease. This feedback
loop means that even if water is available to a plant, if
transpirational forces are great, then the plant will close the
stomata and there will be no photosynthesis even on days
when it would appear that conditions were appropriate.
Plants also tend to have the majority of their stomata on
the undersides of leaves, such that the probability of the
stomata being clogged by particulate matter is reduced. As
we saw before, plants also control indiscriminate water
losses by the presence of a waxy cuticle and short, hair-
like growths from the epidermal layer. These hairs act to
increase the relative humidity near the leaf surface by reduc-
ing the rate of air flow over the leaf surface; a higher relative
humidity decreases transpiration.
The closure of stomatal guard cells to limit transpiration
when water potentials become increasingly more negative
also decreases the potential for food production. Although
more water can be taken up by the larger surface area of root
hairs than lost by leaves if water is unlimited, these
conditions are not typical and other mechanisms are needed
to regulate water losses from the leaves by transpiration
during photosynthesis. If the driving force of water uptake
in the roots is roughly equal to water lost in the leaves,
assuming that water is not limiting, the question becomes
what are the various resistances to the transpiration of water
and what are their magnitudes? Regulation of transpiration
occurs through changes in stomatal conductance. For many
plants, if the leaf water potential drops to

emitted water vapor. Transpiration, T , is related to the

resistances by

T ¼ P wvð leaf Þ P wvð air Þ = r s þ r b ;

(3.8)

where the vapor pressure gradient is P wv (leaf)

P wv (air), in
kilopascals (kPa), r s is the stomatal resistance, and r b is the
boundary layer resistance present at the surface of the leaf.
Because diffusion is a slow process, transpiration is limited
by the magnitude of the diffusional release of water from the
leaf air to the external air.
In most cases the highest resistance to flow from the
leaves is the loss of water vapor. Leaves are attached to the
stems of plants, either directly (from the Latin sessile , mean-
ing sitting on) or indirectly by a leaf stalk, or petiole (from
the Latin petiolus , meaning stalk). The petiole permits the
leaf to move in response to winds without being torn and also
to change the position of the flat surface of the leaf in
reaction to the changing position of the sun with respect to
the relatively fixed location of the plant. Leaves can be
arranged as single leaves from each petiole, called simple
leaves, or as multiple leaves on a single petiole, called
compound leaves. Plants with compound leaves permit
sunlight to reach lower leaves.
Most plants balance the need to have a maximum surface
area exposed to sunlight for photosynthesis and the uptake of
gaseous CO 2 , while reducing the loss of water vapor. In
between the outer layer of the leaf, or epidermal cells, are
the mesophyll cells (from the Greek, meso , meaning mid-
dle). The mesophyll cells contain chloroplasts where photo-
synthesis occurs following photon absorption. The
chloroplasts are mobile within the cell cytoplasm and orient
toward the sun (Fig. 3.15 ). These photosynthetic cells, or
parenchyma, are present in two layers—the palisade and
spongy parenchyma. As the names imply, the palisade cells
are long and tightly spaced and closest to the upper epider-
mis, and the spongy layer below the palisade cells is
composed of less tightly spaced cells surrounded by air
spaces that, as we will see, include water vapor. This is
why for most leaves, the upper surface is a darker green
then the underside, for there are more chloroplasts in the
upper mesophyll cells. The palisade layer is more photosyn-
thetically active than the spongy layer because of

1 MPa, the
stomata close. The water potential difference between the
soil and air is what drives water use by plants. A simple
estimate of the magnitude of this potential for flow is called
stand conductance, in which the daily water use is divided by
the vapor pressure gradient (Rural Industries Research and
Development Corporation 2000).
Transpiration from leaves is controlled by at least two
main factors—the vapor pressure gradient or vapor pressure
deficit (VPD, Pallardy and Kozlowski (1979)) that exists
between that in the leaf air spaces relative to that of the
surrounding air and any resistance to this diffusional transfer
of vapor that might be present. Such resistances include
changes in the stomatal aperture, or stomatal resistance,
where resistance is the inverse of conductance, such that
resistance

its

location.
As might be expected, not all leaves of a plant are
exposed to sunlight under the same intensity. Hence, it
would appear that shaded leaves do not transpire to the
extent that sunlit leaves do. Does this indicate that more
water will become available to the leaves exposed to sun-
light at the expense of decreased water availability to shaded
leaves? Experiments done in the field by Brooks et al. (2003)
indicate that no such response occurred, and that stomatal
conductance in exposed leaves did not increase.

1/conductance. A high conductance is equal
to a low resistance and vice versa. Resistances also include
the boundary of air near the surface of the leaf that receives

¼